According to the technical development and increased demand for mobile devices, the demand for secondary batteries as an energy source is dramatically increasing. Among these secondary batteries, a lithium secondary battery which has higher energy density and voltage, a longer cycle life and a lower self-discharge rate has been commercialized and widely used.
However, a lithium secondary battery has the drawback of a drastic decrease in lifespan because of repeated charging/discharging. Particularly, such a problem becomes more serious at a high temperature. This is because an electrolyte is degraded or active materials are deteriorated due to moisture in the battery or other reasons, and the internal resistance of the battery is increased.
Therefore, currently, the most actively developed and used positive electrode active material for a lithium secondary battery is layered LiCoO2. While LiCoO2 is most widely used because of excellent characteristics of a lifespan and charge/discharge efficiency, due to low structural stability, it has a limitation to be applied to technology for making a high capacity battery.
As a positive electrode active material for replacing the conventional material, various lithium composite metal oxides such as LiNiO2, LiMnO2, LiMn2O4, LiFePO4, Li(NixCoyMnz)O2, etc. have been developed. Among these, LiNiO2 has a battery characteristic of high discharge capacity as an advantageous effect, but it is difficult to be synthesized by a simple solid-phase reaction and has low thermal stability and cycle characteristic as adverse effects. In addition, while a lithium manganese-based oxide such as LiMnO2 or LiMn2O4 has excellent thermal stability and a cheap price as advantageous effects, it has a low capacity and a poor high temperature characteristic as adverse effects. Particularly, LiMn2O4 is commercialized as some of low-price products, but does not have a good lifespan characteristic because of structural deformation (Jahn-Teller distortion) caused by Mn3+. Moreover, LiFePO4 has a low price and excellent stability and thus is recently used in various studies for hybrid electric vehicles (HEVs), but it is difficult to be used for other applications because of low conductivity.
Due to the above-mentioned reasons, recently, the most highly appreciable material as an alternate positive electrode active material for LiCoO2 is a lithium nickel-manganese-cobalt oxide, Li(NixCoyMnz)O2 (where each of the x, y, and z is an atomic fraction of an independent oxide composition element, 0<x≤1, 0<y≤1, 0<z≤1, and 0<x+y+z≤1). This material has a lower price than LiCoO2 and the use in high capacity and high voltage as advantageous effects, but has low rate capability and a low lifespan characteristic at a high temperature as adverse effects.
Therefore, there is an eager demand for a method of preparing a positive electrode active material that can improve performance of a lithium secondary battery by the change of the composition in a lithium composite metal oxide or the control of a crystal structure therein.